The risk of encountering 'unexpected difficulties' in longwall coalmining - risk assessment, risk insurance and risk reduction

In many countries where deep coal is mined, the longwall method is the usual method of extraction. About half the world's coal is mined in this way. Shallower coal may be extracted by room and pillar methods, or even by opencast methods if the coal is very close to the surface. In the United States there is very little longwall mining because there is an abundance of easily accessible coal at shallower depths, where longwall methods are unnecessary. In many other countries, particularly in Europe, the Soviet Union and the People's Republic of China, most of the available coal is at depths where longwall mining has proved to be essential for extraction. The longwall method of mining is very inflexible and very sensitive to interruptions in coal seam continuity. Whenever production of coal is stopped by a hitherto unknown break in coal seam continuity, it is usually attributed to 'unexpected geological difficulties'. The extent to which such difficulties could have been anticipated depends on how well the geology of the coal reserves is known. In order to avoid, or minimize, the risk of encountering such difficulties, it is necessary to explore the geology in the target area in sufficient detail before planning the extraction. This point is so obvious that one might think it need not be made. However, the history of mining shows a great reluctance on the part of mining engineers to carry out the required exploration in advance of mine planning. Some exploration is usually done to prove the reserves, of course, but this is not normally detailed enough to define precisely the geological constraints on mining operations. These have tended to be discovered in the course of mining, and have sometimes caused economic failure of the mine. We can learn from our mistakes. This paper describes the effects that so-called 'unexpected geological difficulties' can have on longwall mining operations, using examples from British coalfields. It also discusses how insurance is normally made against the risk of encountering such difficulties, how the risk can be calculated to determine how much insurance should be carried, and how both the risk and the insurance may be reduced, in certain geological provinces, by exploration.